The present invention comprises an addition to an improvement in one of the important elements of the curtain-type wrapping and banding machines comprising the subject matter of applicants' prior U.S. Pat. Nos. 3,775,939, issued Dec. 4, 1973 and 3,866,389, issued Feb. 18, 1975. The inventions covered by said patents as well as the invention comprising the subject matter of said aforementioned related co-pending application, Ser. No. 523,883, filed Nov. 14, 1974, pertain to extending either bands or sheet films of heat shrinkable plastic material around certain objects such as rolls of sheet material, such as wrapping paper and many other types of sheet material, so as either to enclose such objects circumferentially with such sheet films or all bands which are disposed curtain-like across the path of oncoming articles to be enclosed and then sealing said sheet film or bands around the articles, separate the enclosing material from the supply of the same and then re-constitute the curtain arrangement of the enclosing material.
The foregoing operation of sealing and re-constituting the enclosing sheet film or band type material, which is also thermoplastic, comprises an elongated anvil bar which extends transversely across the machine adjacent the path of movement of the articles to be encircled with the material and a heater bar of similar length which also extends transversely across the machine adjacent said path of the articles and mechanism is included in the machines to move at least one of said bars toward the other or, if desired, move both of said bars toward and from each other to effect such sealing and re-constituting of the thermoplastic material after certain lengths thereof have been encircled around the objects. When bands of heat shrinkable material are employed, they are used to prevent uncoiling of the material from rolls thereof which are sequentially passed through the machine and when sheet film material is employed, it usually extends at least to the opposite ends of the rolls as well as beyond so as to form a wrapper for the object such as coiled rolls of sheet material.
The heater bar usually carries an elongated electrical resistance element such as a wire or narrow strip which extends along the operative face of the heater bar which is in opposition to the operative face of the anvil bar, the resistance element extending substantially the full length of the heater bar. Electric current is supplied to the resistance element by specialized equipment comprising an electrical thermal impulse generator, one popular type of which is manufactured and sold by the Vertrod Corp. of Brooklyn, N.Y. Said generator comprises a series of current transformer and timer units arranged in suitable circuitry within the generator. The function of such generator is to "fire" an impulse of relatively high amperage through the resistance element at the time it is brought into engagement with films or strips of thermoplastic synthetic resin film sheets or bands which extend across the anvil bar so as simultaneously to seal overlying portions of the films or strips which encircle objects around which they extend, sever the encircling material which has just been sealed and immediately re-constitute the severed ends of the material from the supply thereof so as to reestablish a curtain-type arrangement of either the film sheets or bands of material which are engaged by the next oncoming object to be encircled thereby.
As a result of continuous use of the aforementioned apparatus to seal, sever and re-constitute the sheet film or bands of heat shrinkable plastic material comprising synthetic resin, there is a tendency for residual heat to accumulate in the heater bar and the immediate ambient atomsphere which has been found to result in expansion of the electrical resistance element on the heater bar and even produce a limited amount of sagging in the same. Further, the increase in temperature resulting from such continuous use also increases the temperature at which the sealing, severing and re-constituting is effected and this phenomenon causes reduction in operating time due to having to allow an additional time segment in the foregoing operation to permit the thermoplastic material to cool incident to forming a union of the edges of the sheet film or strips incident to re-constituting the same, as well as to effect a satisfactory seal of the material around the articles enclosed thereby. In some circumstances, it even has been necessary to provide auxiliary cooling equipment to effect such seals and reconstituted unions of the materials.
In an effort to overcome the foregoing difficulty, several attempts have been made to provide control equipment to limit the supply of heating current to the electrical resistance elements of the sealing means, the control being responsive to the amount of expansion and contraction of the resistance element. One such example comprises the subject matter of U.S. Pat. No. 3,299,251, dated Jan. 17, 1967, to S. Zelnick and a more recent example is the subject matter of U.S. Pat. No. 3,840,722, issued Oct. 8, 1974, to Rolf Mayer et al. The Zelnick apparatus includes a somewhat complex fluid system employing oil from a reservoir and includes flow control valves, bypasses and the like as well as one way ball check valves which control the operation of electrical contacts which comprise switch means to control the flow of current to the electrical resistance wire of the heater unit. The Mayer et al. patent includes an arrangement whereby the electrical resistance element has one end anchored to a spring to maintain the element taut incident to the expanding and contracting thereof but in doing so, the element is stressed by a spring which is stated in the specification to be a compression spring and, as such, it is directly dependent upon the switch structure, which is not shown. Hence, the sensitivity of adjustment is directly related to the contacting action within the switch which as described in Zelnick, is a poor arrangement which lacks sensitivity.
The desired objective relative to controlling the amount of the ambient heat as well as the limits of expansion and contraction of the electrical resistance element in the sealing mechanism of wrapping and banding machines of the type referred to above is to minimize the amount of current supplied to the resistance element while maintaining the same taut, the supply of current to the heating element being controlled by switch means which is responsive to extremely small amounts of expansion and contraction so as to render the control mechanism extremely sensitive to such very small increments of increase or decrease in the length of the resistance element. An ideal arrangement to accomplish this is provided by the present invention, details of which are set forth hereinafter.